Nasa has released a guide to help the public interpret satellite images of the Earth and understand what specific features mean
For years they were restricted to the secretive world of spies, but with Google Earth and the release of pictures from space by Nasa and other space agencies, satellite images are now available to everyone.
Satellite images can now reveal how our cities are changing in real time, how crops are growing and, as witnessed in the Philippines, when a storm is coming – but only if you know what to look for.
But really understanding the pictures and unlocking their rich stream of information is a complex task.
1) Look for a scale:
Most of us will find the first thing we want to do when looking at a satellite image is identify the places familiar to you.
Some images from military or commercial satellites are detailed enough to reveal, for example, homes, schools and places of work. The satellites zoom in on small areas to collect fine details but in the process sacrifice the big picture.
NASA satellites take the opposite approach. Earth science researchers typically want a wide-angle lens to see whole ecosystems or atmospheric fronts.
Like digital photographs, satellite images are made up of pixels. The most detailed NASA images show 10 meters in each pixel.
Geostationary weather satellites, which observe a whole hemisphere at a time, are much less detailed, seeing one to four kilometers in a pixel.
You can learn different things at each scale. When tracking a flood, a detailed, high-resolution view will show which homes and businesses are surrounded by water. The wider landscape view shows which parts of the county are flooded and where the water is coming from.
2) Look for patterns, shapes, and textures:
Bodies of water – rivers, lakes, and oceans – are often the simplest features to identify because they tend to have unique shapes and they show up on maps.
Farms usually have geometric shapes – circles or rectangles – that stand out. According to NASA, a straight line anywhere in an image is almost certainly human-made.
Geology shapes the landscape in ways that are often easier to see in a satellite image. Volcanoes and craters are circular, and mountain ranges tend to run in long, sometimes wavy lines. Geologic features create visible textures. Canyons are squiggly lines framed by shadows. Mountains look like wrinkles or bumps.
These features can also affect clouds by influencing the flow of air in the atmosphere. Mountains force air up, where it cools and forms clouds. Islands create turbulence that results in swirling vortices or wakes in the clouds. When you see a line of clouds or vortices, they provide a clue about the topography of the land below.
3) Define the colours:
The colours in an image will depend on what kind of light the satellite instrument measured. True-colour images use visible light – red, green and blue wavelengths – so the colours are similar to what a person would see from space. False-colour images incorporate infrared light and may take on unexpected colours. In a true colour image, common features appear as follows:
Sunlight reflecting off the surface of the water makes the water look grey, silver, or white. This phenomenon, known as sunglint, can highlight wave features or oil slicks, but it also masks the presence of sediment or phytoplankton.
Plants come in different shades of green, and those differences show up in the true-colour view from space. Grasslands tend to be pale green, while forests are very dark green. Land used for agriculture is often much brighter in tone than natural vegetation.
4) Find north:
When you get lost, the simplest way to figure out where you are is to find a familiar landmark and orient yourself with respect to it. The same technique applies to satellite images. If you know where north is, you can figure out if that mountain range is running north to south or east to west, or if a city is on the east side of the river or the west.
5) Consider your own knowledge:
Perhaps the most powerful tool for interpreting a satellite image is knowledge of the place. If you know that a wildfire burned through a forest last year, it’s easy to figure out that the dark brown patch of forest is probably a burn scar, not a volcanic flow or shadow.
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